This application claims priority to Japanese Patent Application Number JP2002-054836 filed Feb. 28, 2002, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a photolithography apparatus and a method of controlling a focusing lens, and, more specifically, the present invention relates to a photolithography apparatus using a solid immersion lens and a method of controlling a focusing lens.
2. Description of the Related Art
In recent years, reduction in spot diameter has been required corresponding to the demands for increased memory density of optical disks. In order to do so, a solid immersion lens (SIL), which is shaped like a spherical lens having a part removed with a high refractive index, is disposed between an objective lens and an exposure substrate to obtain larger numerical apertures (for example, a numerical aperture of two or more) than the numerical apertures of the objective lens. The SIL is disposed in such a manner that the spherical surface faces toward the objective lens, and the opposite side from the spherical surface, that is, the flat surface of the lens, faces toward the exposure substrate. The large number of apertures may be achieved also by using a solid immersion mirror (SIM) instead of the SIL.
In order to perform exposure by using the SIL, it is necessary to focus an exposure laser beam passing through the objective lens onto the SIL, and to reduce the distance between the distal end surface, that is, the lowermost end surface, of the SIL (the surface of the SIL facing toward the exposure substrate) and the irradiated surface of the exposure substrate to an area in which evanescent light is generated, in other words, to the so-called near field. In this case, it is necessary to perform gap control to keep the distance between the distal end surface of the SIL and the irradiated surface of the exposure substrate constant.
In order to perform gap control, a method of detecting the distance between the distal end surface of the SIL and the irradiated surface of the exposure substrate, that is to say, the length of the gap, is required. As one of such methods, there is a total reflection detecting method. In this total reflection detecting method, a high NA (numerical apertures) beam (NAxe2x89xa71.0) incident on the focusing lens, including the SIL and the objective lens, is totally reflected from the distal end surface of the SIL when the distal end surface of the SIL is substantially distant from the irradiated surface of the exposure substrate, while the intensity of the return beam is reduced when the distal end surface of the SIL is in the near field.
In other words, gap control is performed in such a manner that the gap length, that is, the distance between the distal end surface of the SIL and the irradiated surface of the exposure substrate, is maintained constant by controlling a gap with a servomechanism when the intensity of the return beam from the distal end surface of the SIL is reduced to a predetermined value.
Gap control utilizing the total reflection detecting method has been performed by disposing a light shielding circular mask in the optical path for blocking returned components of the beam incident on an area of the focusing lens where the NA is less than one and reflected from the distal end surface of the SIL, and detecting only the light intensity of returned components of the beam incident on an area of the focusing lens where the NA is greater than one, in other words, the return beam incident on the distal end surface of the focusing lens at angles not smaller than the critical angle.
Such a method is satisfactory when the gap is controlled by the servomechanism of the focusing lens and the distance between the distal end surface of the focusing lens and the irradiated surface of the exposure substrate is maintained constant. However, when the servomechanism for controlling the gap is not working, a return beam reflected under the principle of Fresnel reflection, excluding returned components of the beam incident on the region of the focusing lens where the NA is greater than one, cannot be blocked sufficiently only by the light shielding circular mask. Consequently, the components of the beam that could not be blocked are superimposed on the detected beam, which results in detection of errors. In addition, leakage of the beam from the edge of the light shielding circular mask may also result in errors in the detected beam.
Accordingly, it is an object of the present invention to provide a photolithography apparatus which resolves the above-mentioned problems.
It is another object of the invention to provide a method of controlling a focusing lens which resolves the above-mentioned problems.
According to the invention, there is provided a photolithography apparatus including a laser beam source, a focusing lens, a detecting unit, and a control unit. The laser beam source emits a laser beam. The focusing lens is disposed in the near field of the exposure surface of an exposure substrate and receives the laser beam emitted from the laser beam source. The detecting unit detects one linearly polarized component of the laser beam reflected from the distal end surface of the focusing lens. The control unit controls the distance between the distal end surface of the focusing lens and the exposure surface based on detection signals detected by the detecting unit.
According to the invention, there is provided a method of controlling the focusing lens. This method controls the distance between the distal end surface of the focusing lens and the exposure surface by the steps of detecting one linearly polarized component of the laser beam reflected from the distal end surface of the focusing lens, which is disposed in the near field of the exposure surface of the exposure substrate, and moving the focusing lens in the direction of the optical axis based on the signal obtained by detection.